Physical and thermomechanical characterization of unidirectional Helicteres isora fiber-reinforced polylactic acid bio-composites

Identifying novel cellulose fiber bio-composites has become a vital initiative in the exploration of sustainable materials due to increased global concern for the environment. This growing focus on eco-friendly materials has gathered significant attention in recent years. The current investigation deals with one such material, Helicteres isora reinforced Polylactic acid composites. Surface chemical treatment of fiber is one of the most effective methods to modify the hydrophilic fiber to increase its compatibility with the polymer matrix. Sodium hydroxide was used as a pre-treatment chemical to remove any impurities from the fiber surface. Pre-treated fibers were treated with Methacryl silane and Potassium permanganate solution to chemically modify the fiber surface. Density, void content and water absorption behavior of the composites were analyzed as per the standard procedure. Tensile and flexural tests were conducted to evaluate the mechanical strength, modulus, and flexibility of the unidirectional composites. Thermogravimetric and differential thermal analyses were performed to investigate the thermal stability, melting behavior and degradation profiles of prepared composites. A study of failure mechanisms and morphology of the fractured surface through photographs and SEM images revealed fiber splitting and delamination as the dominant reasons behind the failure of composites under tensile loading. Silane-treated Helicteres isora fiber-reinforced Polylactic acid composite exhibited lower water absorption and higher tensile strength than its counterparts. Untreated fiber composite showed maximum flexural strength among the tested composites. By collectively evaluating the results of the tests and properties of the composites, silane-treated fiber-reinforced Polylactic acid composites stands out as the most favorable choice.


Materials and methods
The fabrication of untreated and chemically treated Helicteres isora fiber-reinforced PLA composites was done by the combination of two processes.The solution casting technique was used to prepare prepregs of Helicteres isora fiber mats coated with PLA, followed by compression moulding with a heating facility.The detailed steps or stages of the methodology are presented visually in Fig. 1, and each step is discussed in detail and explained in subsequent sections.

Materials
Raw bast fibers from the Helicteres isora plant were procured from the local forest in southwestern coastal India, and the clean fibers from well-grown branches were extracted by water retting for 15 days, which is also followed in the published work 13 .Retted Helicteres isora fibers were carefully separated and cleaned with running water until the visible impurities were removed.The diameter of Helicteres isora fiber was in the range of 90 to 120 microns as per the measurements done using the toolroom microsope (Mitutoyo TM-510).The microfibrillar ngle of the fiber is 20°-26°, and the length-to-diameter ratio of ultimate cells is 99 14 .With a fiber Further, the fibers were treated with chemicals for surface modifications.Alkali Sodium hydroxide (NaOH) is used as a preliminary treatment and further treated with Methacryl silane (Silane) and Potassium permanganate (PP) solutions.
In the current study, Ingeo biopolymer, 2003D grade PLA is used as matrix material, a plant-based biocompostable thermoplastic supplied by Natur Tec India Pvt.Ltd, Chennai, India.PLA is the most widely used bio-polymer in composite applications because of its better physical and mechanical properties than other starchbased thermoplastics 25 .Detailed specification of PLA is given in Table 1 as provided by the supplier.

Chemical treatments
Raw Helicteres isora fibers were made to undergo a series of treatments to enhance their properties.Initially, they were immersed in a 5% w/v NaOH aqueous solution to eliminate impurities on the fiber surface, with a soaking duration of 60 min 13 .Following the NaOH treatment, two separate chemical treatments were performed using Silane and PP solutions.One batch of NaOH pre-treated Helicteres isora fibers was treated with a 2% v/v Methacryl silane solution in a mixture of ethanol and deionized water (ratio 60:40) for 60 min 26 .Another batch was treated with a 0.5% w/v aqueous solution of PP for 3 min 27 .After each chemical treatment, the fibers were rinsed thoroughly with distilled water until achieving a neutral pH of the rinsed water.These untreated, NaOH, Silane and PP solution-treated fibers were used to prepare individual composites with PLA as a matrix.

Fabrication of Helicteres isora fiber-reinforced PLA laminated composites
The combined method of fabrication, which comprises solution casting to impregnate PLA into Helicteres isora fiber mat followed by compression moulding technique, is used for preparing composite specimens.Combining solution casting and compression molding techniques unveiled an adaptable and novel approach to fabricating Helicteres isora fiber-reinforced PLA composites.The stages of fabrication are explained in detail in the following sections.

Preparation of prepregs using solution casting technique
The mat of Helicteres isora fiber was prepared by gluing the procured fiber layers side by side, as shown in Fig. 2a, with the dimension of 160 mm × 250 mm for processing with PLA solution.The average value of fabric density for the formed fiber mat was found to be 75-80 GSM.The PLA solution was prepared using dichloromethane (methylene-di-chloride or MDC) by blending a specific quantity of PLA granules into the MDC solvent as in Fig. 2b 28 .This resulting liquid PLA solution served as liquid resin for preparing matrix-coated Helicteres isora fiber mats with a 50% fiber volume fraction 16 .A glass tray was used for solution casting to impregnate or coat Helicteres isora fiber mats with PLA, as shown in Fig. 2c.The desired amount of PLA solution was poured and spread evenly in the glass tray and the fiber mat was placed on it.One more layer of PLA solution is spread evenly on the fiber using a soft brush, allowing it to cure.While curing, MDC evaporates completely and the PLA coats onto the fiber mat, which is shown in Fig. 2d.Similarly, multiple fiber mats were prepared to fabricate untreated and chemically treated Helicteres isora fiber and PLA laminated composites.
Along with the prepreg preparation, a clear PLA film of a thickness of 100 microns was prepared using the solution casting technique, and the same has been tested for Tensile and Thermal properties.

Preparation of laminates using compression moulding
Prepregs of Helicteres isora fiber mats prepared were stacked and placed in the steel mould, as shown in Fig. 3a,b.The steel mould with fiber mats as in Fig. 3c was kept under a pressure of 2 MPa in the compression moulding machine (Modern Hydraulics Compression Moulding machine), which is demonstrated in Fig. 3d, maintaining at a temperature of 150 °C for 15 min 29 .Subsequently, the laminate was left within the mold, still under pressure, to cool and cure for 24 h.Later, the laminate was removed from the mould and stored at room temperature with dead weight for another 24 h to cure completely.The fabricated specimens in this method are shown in Fig. 4. Untreated, NaOH, Silane and PP-treated fiber-reinforced PLA composites are abbreviated as UT, AT, ST and PT respectively, and the same notations are used in all upcoming sections.

Physical properties of the composites
The experimental density of the individual specimens was determined by measuring the weight and volume of the specimen.The laminates were cut into specimens of 20 mm × 20 mm size to conduct physical properties tests.The theoretical density, on the other hand, is computed using the rule of the mixture (ROM), as shown in Eq. (1), and the void fraction is calculated using the theoretical and actual densities of the composites.The void content of the composite prepared was calculated using Eqs.( 2) and (3), as per ASTM-D2734.
where: ρ c is the density of composite, ρ f is the fiber density, ρ m is the matrix density, and f is the fiber volume fraction.
where: T is the theoretical density, R is the matrix weight %, D is the density of matrix material, r is the fiber weight %, and d is the density of fiber (1) where: V is the void content, volume %, T d is the theoretical composite density and M d is the experimental density.
The water absorption ability of the composite specimens was analyzed using the immersion method.The specimen was immersed in distilled water, and the weight gain was recorded for predefined intervals of time.Weight gain by the specimen is plotted against the square root of time in seconds.Equation ( 4) illustrates the relation between mass gain and diffusion coefficient calculated using Eq. ( 5) 30 .
where: M t is the mass gain at time 't' (g), M ∞ is the mass gain at t = ∞ in (g), t is the time (s), l is the half thickness of the specimen (m), θ is the slope of M t vs √ t from the graph.
(  www.nature.com/scientificreports/Mechanical testing of specimens Specimens for tensile, flexural and void fraction or density measurement were cut from the laminates as per the established testing standards.Tensile specimens (of dimension 200 mm × 15 mm × 2 mm) were cut as per ASTM D 3039 test method as in Fig. 5a-d.The tensile test was conducted using a BiSS universal testing machine with a load cell capacity of 50 kN, and a specimen-mounted setup is shown in Fig. 5e.The test was conducted at a speed of 2 kN per minute for each specimen with a data sampling rate of 30 per second.Flexural specimens ( of dimension 127 mm × 12.7 mm × 2 mm) were cut as per ASTM D790 as shown in Fig. 6a-d.The flexural test was conducted using a Zwick Roell universal testing machine, a setup for polymer testing as shown in Fig. 6e, the test speed kept was 1 mm/min.

Thermal characteristics
Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) were performed on each composite using MAS-5800 instrument with a heating/cooling rate of 10 °C/min.TGA and DTA curves of raw Helicteres isora fiber, neat PLA specimen, UT, AT, ST and PT composite specimens are discussed in Sect.3.3.

Density, void content and water absorption behaviour of composites prepared
Theoretical densities of UT, AT, ST and PT composites based on ROM were calculated.The void fraction is calculated based on these values of theoretical densities of the composites.The densities and void fractions in percentages of the composites are shown in Table 2.The average void fraction of the tested specimen was less than 5% in all the cases.Least density was found in UT composite specimen (1.204 ± 0.012 g/cm 3 ) among its counterparts, and PT specimen showed the least void content (3.45 ± 1.130%), followed by ST specimen (3.96 ± 0.840%).Due to the shrinkage of fiber upon alkali treatment, the density of the composite made from alkali-treated fiber increased.In permanganate treatment, the fiber density is reduced due to the rough surface formed, resulting in the reduced density of the composite made with permanganate-treated fiber 31 .
The water absorption tendency of the specimens is plotted in the graph as mass gain % versus square root of time in seconds, as shown in Fig. 7.The diffusion coefficients of specimens are listed in Table 3.The diffusion coefficient plays a crucial role in understanding and predicting the water absorption behavior of polymer  www.nature.com/scientificreports/composites.It is a quantitative measure that reflects the rate at which water molecules move within the composite material.Understanding the diffusion coefficient allows for the prediction of water uptake over extended periods, which is essential for applications where long-term durability and dimensional stability are of paramount importance 32 .A high content of cellulose is responsible for fibers' significant moisture absorption.Abundantly available polar hydroxyl groups within the cellulose crystalline region impart hydrophilicity to the natural fibers 33 .
In the current work, the constituent analysis of raw Helicteres isora fiber using the gravimetry method revealed a remarkable cellulose content of up to 70% in the raw fiber, which is comparable to 74% cellulose content in raw Helicteres isora fiber as claimed in the published literature 14 .Silane treatment significantly reduces the fiber's susceptibility to water ingress by eliminating surface hydroxyl groups by chemically modifying the fiber surface, converting hydrophilic hydroxyl groups into water-resistant bonds 34 .The water absorption ability of the material also depends on the void content present in the material.Water absorption in the specimens UT and AT with higher void content was more compared to lower void content specimens ST and PT.As the void content decreases due to effective fiber treatment, the diffusion coefficient of the composite also tends to decrease.This is because the presence of voids facilitates water uptake and transport within the material.When void content is minimized, the water molecules encounter more resistance as they traverse the composite, leading to a lower diffusion rate.Consequently, the composite exhibits reduced water absorption, which is advantageous

Mechanical characterization
Tensile test Tensile characterization is essential for evaluating the strength and elongation behavior of materials under linearly applied forces.The incorporation of Helicteres isora fibers into PLA matrices introduces a natural reinforcement that can significantly impact the tensile properties of the resulting composites.The tensile behavior of UT, AT, ST and PT composite specimens are illustrated in Fig. 8. Tensile properties of Raw Helicteres isora fiber were determined as per ASTM D3822 standard.The tensile properties of raw fiber, neat PLA, UT, AT, ST and PT composites are listed in Table 4. Based on the obtained results, the Methacryl silane-treated Helicteres isora fiber-reinforced PLA composite (ST) exhibited the highest tensile strength, which is 153.15 ± 0.65 MPa.This is attributed to the bipolar coupling action of Silane, establishing a firm compatibility between the Helicteres isora fiber and the PLA matrix.Specimen ST has shown the highest strength and better modulus among all alternatives.The ST specimen exhibited lower brittleness, resulting in a lower tensile modulus (15.19 ± 0.06 GPa) compared to the modulus of PT specimen (15.85 ± 0.35 GPa), which had higher brittleness than other specimens.This argument is well supported by the failure mechanisms discussed in the upcoming Sect.3.2.2.
The tensile strengths of some established natural fiber-reinforced PLA composites are compared with Helicteres isora fiber-reinforced PLA composite in Table 5.The strength and modulus of Helicteres isora fiber-reinforced PLA composite are superior to many other natural fiber-reinforced PLA composites.

Failure mechanisms in tensile test
Failure mechanisms in natural fiber-reinforced polymer composites can result from various factors and interactions between the polymer matrix and the reinforcing natural fibers.A few of them are matrix cracking, fiber pullout, fiber fracture, interfacial debonding, and delamination of fiber layers.Helicteres isora fiber is a moderate to high modulus fiber with moderate to high tensile strength.According to ASTM D3039 testing method, failure  www.nature.com/scientificreports/modes in high-modulus fiber include explosive failure, long splitting along the fiber direction, and delamination of fiber layers.(a)-(d) shows the failure modes of UT, AT, ST and PT tensile specimens.UT specimen shows splitting and brittle fiber fracture during the tensile test.The explosive type of failure within the specimen's gauge length was observed in AT.This mode of failure is due to the high modulus of alkali-treated fiber.There are visible signs of fiber splitting, which suggests that the individual fibers within the composite split apart during the failure, and the delamination occurred at the edge.The composite ST tensile specimen has shown smaller delamination of the fiber-matrix interface in the fractured surface; this shows that the ST specimen has lower brittleness than its counterparts.Specimen of PT composite has shown broom like failure; this particular mode of failure was attributed to the occurrence of fiber splitting at various locations within the specimen gauge length.Before the ultimate and complete failure of the specimen, there was a process of delamination taking place at the surface layer.This delamination, or separation of layers, was a crucial precursor to the subsequent failure [40][41][42][43] (Fig. 9).
The SEM image of a fractured surface of the UT composite specimen shown in Fig. 10a depicts the brittle fracture of the Helicteres isora fiber.Which in turn illustrates the low ductility of the composite.SEM image of the fractured end of the AT specimen, as illustrated in Fig. 10b, shows the fiber splitting and matrix debris of the fiber-matrix interface.The surface of the fibers exhibited the presence of matrix debris, indicating that the matrix material had separated from the fibers during the course of the failure.SEM image of the ST composite specimen's fractured section shown in Fig. 10c highlighted the crack formation during the failure and the brittle fracture of fibers perpendicular to their direction.It is observed that the specimen exhibited improved fiber-matrix adhesion, as evidenced by the limited occurrence of fiber delamination in a small segment in comparison to the other test specimens.PT composite specimen's fractured surface SEM image is shown in Fig. 10d, which revealed the fibers themselves displayed characteristics of brittle fracture, suggesting that they had fractured with limited plastic deformation.This brittle failure behavior of the fibers is a key aspect of the overall failure mechanism.

Flexural properties of the composites
The flexural behavior and properties are illustrated in Fig. 11a,b.One of the key observations from this illustration is that the flexural strength of the untreated Helicteres isora fiber composite surpasses that of the chemically treated fiber composites.This finding can be attributed to the specific changes that occur in the fibers as a result

Thermal behavior of composites
The superimposed TGA and DTA curves of Raw Helicteres isora fiber, neat PLA, UT, AT, ST and PT specimens are shown in Fig. 12a,b, respectively.Up to 100 °C, the initial weight loss observed can be attributed to the evaporation of moisture and other low-temperature volatile compounds present in the specimens.Beyond this point, the curves for different specimens diverge, indicating variations in thermal stability.In raw fiber samples, the weight loss within 100 °C is more due to the higher moisture content of the fiber 44 .Neat PLA has the least  www.nature.com/scientificreports/weight loss before 100 °C because of its hydrophobicity and low moisture content.In the PT specimen, material degradation commenced at 225 °C.For the UT and AT specimens, this degradation process was slightly delayed, initiating in the temperature range of 250-270 °C.Notably, the ST specimen's curve exhibited a different behavior.In this case, degradation did not commence until 270 °C, and degradation ended in the temperature range of 340-350 °C.This observation indicates that the composite with silane-treated fibers exhibits higher thermal stability up to 270 °C compared to the other composite specimens.In other words, it can withstand higher temperatures without significant degradation or weight loss.
The DTA curves of composite specimens reveal both endothermic and exothermic reactions occurring during the heating process.The peaks, both exothermic and endothermic, along with their magnitudes, signify the thermal phase transformation of the composites.Endothermic reactions involve processes like the evaporation and melting of volatile compounds and polymeric materials.In contrast, exothermic reactions encompass chemical reactions and oxidative degradation.The upward peak in the DTA curve signifies the oxidative decomposition of the material.Notably, within the temperature range of 200-300 °C, there is an endothermic reaction associated with cellulose degradation.This is evidenced by the descending peak observed between 225 and 275 °C, indicating the degradation of cellulose present in the fiber.The weight loss of Helicteres isora fiber, with significant decomposition occurring within the temperature range of 225-375 °C45 .This decomposition primarily stems from the breakdown of hemicellulose, cellulose, and lignin present in the fiber.The decomposition of natural fibers initiates with hemicellulose, followed by cellulose, lignin, and ash.Hemicellulose decomposition typically commences early, around at 220 °C, attributed to its chemical composition featuring a random amorphous structure with minimal strength, thus rendering it readily hydrolyzed 46 .As for the degradation of lignin within the fiber, it initiates at a temperature of 400 °C and persists until reaching 600 °C.The gradual degradation of residual lignin is signified by the slope of the curve after 400 °C47 .
The PLA polymer shows an initial peak at 70 °C, which exhibits the glass transition temperature of PLA.The curve also exhibits a melting point at 200 °C and begins to degrade at 300 °C, ending at 380 °C48 .Each developed composite shows similar behavior in the TGA and DTA curves with a slight shift in the degradation peaks due to the partial removal of hemicellulose and varied thermal stability of the fiber upon chemical treatments.DTA curves of AT, ST and PT specimens show that the major oxidative degradation of material started at 310 °C, 335 °C and 275 °C, and ended after 350 °C, 375 °C and 325 °C respectively.Overall, the results from the DTA curve demonstrate the composite materials' ability to maintain thermal stability at elevated temperatures.As per the results obtained, Silane-treated fiber-reinforced PLA composite shows a more stable behavior before degradation up to 275 °C than all other specimens.

Conclusions
Based on the results obtained from the physical, mechanical and thermal characterizations carried out on the Helicteres isora fiber-reinforced PLA biodegradable composites prepared by using a combined fabrication technique, which is solution casting followed by compression moulding, and based on the literature referred and the correlation of results with the previously published results, following conclusions are drawn.
• Specimen PT exhibited the lowest void fraction among all the prepared specimens.ST specimen exhibits a lower void content than the UT and AT specimens and a slower water absorption tendency than all other specimens.Lower void content in treated (ST) composites resists the diffusion of water molecules, thereby reducing water absorption.• In terms of tensile performance among the UT, AT, ST, and PT specimens, it is evident that the Methacryl silane-treated (ST) composite displayed improved tensile properties by 7% above UT specimens, which indicates that the fiber-matrix bonding has been enhanced by Silane treatment on the fiber surface.Analyzing the flexural behavior of all specimens under loading conditions reveals that the UT specimen exhibits

Figure 1 .
Figure 1.Flow diagram showing the methodology.

Figure 2 .
Figure 2. Solution casting process: (a) Fiber mat prepared, (b) PLA solution in MDC, (c) Solution casting in a glass tray and (d) Solution-casted PLA resin-coated Helicteres isora fiber mat.

Figure 3 .
Figure 3. Process of preparing composites: (a) Prepregs of Helicteres isora fiber mats, (b) Prepregs in a greased steel mould, (c) Mould with top cover and (d) Mould placed in compression moulding machine under pressure.

Figure 7 .
Figure 7. Water absorption tendency of UT, AT, ST and PT composite specimens.

Figure 8 .
Figure 8. Tensile stress-strain curve of UT, AT, ST and PT composites.

Figure 10 .
Figure 10.SEM image of a fracture surface of (a) UT, (b) AT, (c) ST and (d) PT specimen.

Figure 12 .
Figure 12.(a) TGA and (b) DTA curves of Raw fiber, neat PLA, UT, AT, ST and PT composite specimens.

Table 1 .
Specifications of PLA 2003D supplied by Natur Tec India Pvt. Ltd.

Table 2 .
Density and void content of specimens.

Table 3 .
Diffusion coefficient of composites.

M ∞ (g) Half thickness of specimen l (mm) Slope of M t vs √ t Diffusion coefficient D (mm 2 /sec)
for maintaining its mechanical integrity and retarding degradation of the material over time.The present study on water absorption of the composites revealed that the silane-treated fiber-reinforced composite has shown reduced water absorption and a slower rate of water absorption than other composites.

Table 4 .
Tensile properties of raw Helicteres isora fiber, neat PLA, UT, AT, ST and PT composites.

Table 5 .
Comparison of Tensile strength and moduli of PLA-based natural fiber composites.